Simian immunodeficiency virus (SIV) infection of macaque monkeys is an important model for the study of AIDS. The nucleotide sequence similarities to HIV and ability to induce AIDS in macaques make SIVs and their infection of these monkeys particularly important models for the development of AIDS vaccine strategies. In investigating the immune response to SIV in macaques, it will be important therefore to define the genes and gene products of the macaque's MHC. The investigator will therefore first characterize the MHC genes and gene products of the rhesus monkey and, and second explore the possible relationships between certain MHC haplotypes and length of survival after SIV infection in this species.